Option variable
flipflagDefault: false. If false then the indices will be
renamed according to the order of the contravariant indices,
otherwise according to the order of the covariant indices.
If flipflag is false then rename forms a list
of the contravariant indices as they are encountered from left to right
(if true then of the covariant indices). The first dummy
index in the list is renamed to %1, the next to %2, etc.
Then sorting occurs after the rename-ing (see the example
under rename).